1. Field of the Invention
The present invention relates to a semiconductor device and a manufacturing method thereof, and more particularly to a semiconductor device having an insulated gate bipolar transistor and a free wheel diode and to a manufacturing method thereof.
2. Description of the Background Art
In recent years, from a viewpoint of energy saving, an inverter circuit has widely been used for control of home electric appliances and industrial power units and the like. The inverter circuit carries out power control by repeating ON and OFF of a voltage or a current by means of a power semiconductor device. If a rated voltage is 300V or greater, an insulated gate bipolar transistor (Insulated Gate Bipolar Transistor: hereinafter, abbreviated as “IGBT”) is mainly used in the inverter circuit in view of its characteristics.
In many cases, the inverter circuit drives mainly an inductive load such as an induction motor, and in such a case, counter-electromotive force is generated from the inductive load. Accordingly, a free wheel diode for feedback of the current generated from the counter-electromotive force is required. A normal inverter circuit is formed by connecting an IGBT and a free wheel diode in an anti-parallel manner (Reverse Conducting IGBT). In order to achieve smaller size and light weight of such an inverter device, a semiconductor device realized as a single chip by integrating a free wheel diode and an IGBT has been developed.
For example, Japanese Patent Laying-Open Nos. 06-085269, 06-196705, 2005-057235, and the like have conventionally proposed such a semiconductor device realized as a single chip by integrating an IGBT and a free wheel diode.
In the reverse conducting IGBT, an insulated gate field effect transistor portion of an IGBT and an anode region of a free wheel diode are formed on a surface side of a semiconductor substrate, and a collector region of the IGBT and a cathode region of the free wheel diode are formed on a back surface side thereof. Then, a back electrode is formed on the back surface of the semiconductor substrate so as to be electrically connected to both of the collector region and the cathode region.
Conventionally, the back electrode has had such a structure that Al (aluminum), Mo (molybdenum), Ni (nickel), and Au (gold) are successively stacked in this order from the back surface side of the semiconductor substrate. The back electrode Al/Mo/Ni/Au can establish excellent ohmic contact with a p-type impurity. Accordingly, a satisfactory ON voltage can be obtained at the time of conduction of the insulated gate field effect transistor portion of the IGBT.
On the other hand, the back electrode Al/Mo/Ni/Au is less likely to establish excellent ohmic contact with an n-type impurity. Accordingly, the ON voltage is deteriorated at the time of conduction of the free wheel diode.
In addition, in the reverse conducting IGBT, lifetime control should be carried out by irradiating a semiconductor wafer with radiation such as electron beams, γ rays, neutron beams, ion beams, and the like. Moreover, in order to achieve excellent ohmic contact between the back electrode Al/Mo/Ni/Au and the semiconductor substrate, the semiconductor wafer should be subjected to heat treatment after the back electrode is formed.
The semiconductor wafer for the reverse conducting IGBT, however, has a smaller thickness after polishing of the back surface. Therefore, if heat treatment is performed after the back electrode is formed, warpage of the semiconductor wafer is caused due to difference in a coefficient of thermal expansion between silicon and the back electrode, which results in poorer mass productivity.